1. Field of the Invention
The present invention relates to a precursor for fabricating a Nb3Sn superconducting wire by an internal Sn process (hereinafter, which may be referred to as a “precursor for fabricating a superconducting wire”) and a Nb3Sn superconducting wire fabricated using such a precursor. More particularly, the invention relates to a Nb3Sn superconducting wire useful as a material for superconducting magnets and a precursor for the Nb3Sn superconducting wire.
2. Description of the Related Art
Examples of the practical use of superconducting wires include superconducting magnets that are used for high-resolution nuclear magnetic resonance (NMR) analyzers, nuclear fusion devices, accelerators, etc. As the superconducting wires used for superconducting magnets, Nb3Sn wires have been put into practical use. In the fabrication of Nb3Sn superconducting wires, a bronze process is primarily employed. In the bronze process, a plurality of cores made of Nb or a Nb-based alloy are embedded in a Cu—Sn-based alloy (bronze) matrix to form a composite wire. The composite wire is subjected to diameter reduction, such as extrusion or wire drawing, so that the cores have a smaller diameter and formed into filaments (hereinafter referred to as Nb-based filaments). A plurality of composite wires, each being composed of the Nb-based filaments and bronze, are bundled into a wire group. Copper for stabilization (stabilizing copper) is arranged in the periphery of the wire group, and then area reduction is further performed. Subsequently, the wire group, which has been subjected to diameter reduction, is subjected to heat treatment (diffusion heat treatment) at about 600° C. to 800° C. to generate Nb3Sn compound layers at the interfaces between the bronze matrix and the Nb-based filaments.
However, in this process, since the concentration of Sn solid soluble in bronze is limited (15.8% by mass or less), the resulting Nb3Sn compound layers have a small thickness, and the crystallinity is degraded. Thus, it is not possible to obtain a high critical current density Jc, which is disadvantageous. In a superconducting magnet (hereinafter, which may be represented by a “NMR magnet”), as the critical current density Jc of the wire is increased, the NMR magnet can be made more compact, and it is possible to reduce the cost and shorten the delivery time of the magnet. Furthermore, since the area of the superconducting portion in the conductor can be reduced, it is possible to reduce the cost of the wire itself.
In addition to the bronze process described above, an internal Sn process is also known as the process for fabricating a Nb3Sn superconducting wire. In the internal Sn process (which is also referred to as an “internal diffusion process”), unlike the bronze process, since the Sn concentration is not limited due to the solid solubility limit, the Sn concentration can be set as high as possible, and a high-quality Nb3Sn phase can be generated. Therefore, it is believed to be possible to obtain a high critical current density Jc. Furthermore, in the wire fabricated by the bronze process, since work hardening occurs in the Cu—Sn alloy during cold working, many annealing steps are required. In contrast, in the internal Sn process, annealing is seldom required and it is possible to shorten the delivery time. Therefore, use of superconducting wires fabricated by the internal Sn process (hereinafter, which may be referred to as “Nb3Sn superconducting wires by the internal Sn process”) in the NMR magnet application has been desired.
In the internal Sn process, as shown in FIG. 1 (which is a schematic diagram of a precursor for fabricating a Nb3Sn superconducting wire), a core 3 made of Sn or a Sn-based alloy (hereinafter, which may be referred to as a “Sn-based metal core”) is embedded in the center of a Cu or Cu-based alloy member (hereinafter, which may be referred to as a “Cu matrix”) 4, and a plurality of cores 2 made of Nb or a Nb-based alloy (hereinafter, which may be referred to as “Nb-based metal cores”) are arranged so as not to be in contact with each other in the Cu matrix 4 which surrounds the Sn-based metal core 3 to form a precursor (precursor for fabricating a superconducting wire) 1. The precursor 1 is subjected to wire drawing, and then subjected to heat treatment (diffusion heat treatment) so that Sn in the Sn-based metal core 3 is diffused and reacted with the Nb-based metal cores 2 to generate a Nb3Sn phase in the wire. For example, refer to claims, etc. in Japanese Unexamined Patent Application Publication No. 49-114389 (Patent Document 1).
In the precursor described above, as shown in FIG. 2, it is common to employ a structure (precursor 5) in which a diffusion barrier layer 6 is disposed between a portion in which the Nb-based metal cores 2 and the Sn-based metal core 3 are arranged and an external stabilizing copper layer 4a. The diffusion barrier layer 6 is, for example, composed of a Nb layer, a Ta layer, or two layers including a Nb layer and a Ta layer. The diffusion barrier layer 6 prevents Sn in the Sn-based metal core 3 from being diffused to outside during the diffusion heat treatment and has an effect of increasing the purity of Sn in the superconducting wire.
The precursor for fabricating the superconducting wire described above is produced by the method described below. First, a Nb-based metal core inserted into a Cu matrix tube is subjected to diameter reduction, such as extrusion or wire drawing, to form a composite member (usually having a hexagonal cross section), and the composite member is cut into an appropriate length. A plurality of the resulting composite members are inserted into a billet having an external cylinder made of Cu and provided with or without a diffusion barrier layer, and a Cu matrix (solid Cu billet) is arranged in the center thereof. After extrusion is performed, the Cu matrix in the center is mechanically perforated to form a pipe-shaped composite member. Alternatively, in another method, a plurality of the composite members are inserted into a hollow billet, which includes a Cu external cylinder and a Cu internal cylinder and which is provided with or without a diffusion barrier layer 6, (between the external cylinder and the internal cylinder), and pipe extrusion is performed to form a pipe-shaped composite member.
Subsequently, a Sn-based metal core is inserted into the void in the center of the pipe-shaped composite member fabricated by any of the methods described above, and diameter reduction is performed. Thereby, a precursor element including the Nb-based metal cores 2 and the Sn-based metal core 3 as shown in FIG. 1 or 2 is obtained. Hereinafter, such a precursor element may be referred to as a “single element wire”. In FIG. 1 or 2, a single element wire having a structure in which one Sn-based metal core 3 is arranged is shown. However, the structure of the single element wire is not limited thereto, and it is also possible to employ a structure in which a plurality of Sn-based metal cores 3 are arranged.
A plurality of the precursors (single element wires) fabricated as described above are bundled into an aggregate and inserted into a Cu matrix tube provided with or without a diffusion barrier layer 6, and diameter reduction is further performed to form a precursor for fabricating a multi-core superconducting wire (hereinafter, which may be referred to as a “multi-element wire”)
FIGS. 3 and 4 show examples of the structure of a multi-element wire. FIG. 3 shows a multi-element wire 7, in which a plurality of precursors (single element wires) 1, each shown in FIG. 1, are bundled as an aggregate and embedded in a Cu matrix 4 provided with a diffusion barrier layer 6a and a stabilizing copper portion 4a, to constitute a superconducting matrix portion. For example, refer to Teion Kogaku (Cryogenic Engineering) Vol. 39(9), 2004, pp. 391-397 (Non-Patent Document 1). FIG. 4 shows a multi-element precursor 8, in which a plurality of precursors (single element wires) 5, each shown in FIG. 2, are bundled as an aggregate and embedded in a Cu matrix 4 not provided with a diffusion barrier layer, to constitute a superconducting matrix portion. For example, refer to IEEE Transaction on Magnetics, Vol. MAG-19, No. 3, May 1983, pp. 1131-1134 (Non-Patent Document 2).